Duloxetine has been studied in pain due to osteoarthritis (OA) in 2 previous placebo controlled clinical trials. In clinical practice, when nonsteroidal anti-inflammatory drugs (NSAIDs) are ineffective in reducing pain due to OA, clinicians often add a second agent without discontinuing NSAIDs. In this study, we will investigate whether adding duloxetine to NSAIDs provides additional pain relief and functional improvement in patients with knee pain due to OA.